The present invention relates to natural gas fired combined cycle power plants and, in particular, to a modified bottoming cycle for fuel gas saturation and heating to increase power output and thermodynamic efficiency.
In conventional bottoming cycle Heat Recovery Steam Generators (HRSG) there is a large temperature difference between the hot gas and the cold water in the lower pressure economizer (LP-EC) resulting in thermodynamic exergy (thermodynamic potential) losses which limit the power output in the cycle. Heretofore there have been attempts to design bottoming cycles for better temperature matching in the HRSG, such as the Kalina cycle, which uses a multi-component fluid, e.g., ammonia and water, with non-isothermal boiling characteristics. Such multi-component fluid cycles provide better temperature matching in the entire HRSG and efficiency gains. However, significant practical difficulties exist in using multi-component fluids in bottoming cycles.
Fuel heating is currently implemented in some combined cycle power plants for improving thermal efficiency. Although current fuel heating methods result in plant power output reduction, when heating the fuel above the LP steam temperature, the gain in thermal efficiency as a result of the decreased heat consumption makes fuel heating an economically attractive design option. However, there remains a need for a method and apparatus for achieving a better temperature matching in the HRSG while avoiding power plant output reduction.
The bottoming cycle design method according to a presently preferred embodiment of the present invention results in better temperature matching between the hot and cold heat exchange streams below the lowest pressure evaporator temperature by providing a water heating section for fuel gas saturation in parallel with the lower pressure economizer (LP-EC) in the heat recovery steam generator. Thus, the heat source for fuel gas saturation in the current invention is the gas turbine exhaust gases. The increased gas mass flow due to the addition of moisture results in increased power output from the gas and steam turbines. Fuel gas saturation is followed by superheating the fuel, preferably with bottom cycle heat sources, resulting in a larger thermal efficiency gain compared to current fuel heating methods. There is a gain in power output compared to no fuel heating, even when heating the fuel to above the LP steam temperature. As noted above, current fuel heating methods would result in a power output loss compared to no fuel heating. Thus, fuel gas saturation and subsequent super heating with the cycle of the invention results in increased power output and thermodynamic efficiency compared to a conventional combined cycle with fuel heating to the same temperature or a cycle with no fuel heating. This improved performance is a result of the reduced exergy losses in the HRSG with the modified bottoming cycle described.
The invention is thus embodied in a combined cycle system including a gas turbine, a steam turbine, and a heat recovery steam generator, wherein gas turbine exhaust gas is used in the heat recovery steam generator for generating steam for the steam turbine, said gas turbine exhaust gas flowing from an entry end to an exit end of the heat recovery steam generator, and wherein the system further comprises a fuel gas saturator assembly for saturating fuel gas with water and heating the fuel gas, the heat recovery steam generator (HRSG) including a first water heater for heating water with heat from the exhaust gases, to define a heat source for the fuel gas saturator assembly; and a fuel gas superheater for superheating fuel gas that has been saturated and heated by the fuel gas saturator assembly for supply to the gas turbine.
In one embodiment, the fuel gas saturator assembly comprises a fuel gas saturator packed column, for saturating and heating fuel gas with heated water received from the first water heater of the HRSG. In another embodiment, the fuel gas saturator assembly comprises a water inlet for adding water to the fuel gas and a heat exchanger for heating fuel gas saturated with the water input at the water inlet. In this case, the heat exchanger receives and uses the heated water from the first water heater to heat the fuel gas. Whether a heat exchanger or a saturator column is used, in a preferred embodiment of the invention, the fuel superheater heats the saturated fuel gas using a heat recovery steam generator heat source.
The invention is also embodied in a method for increasing power output and thermodynamic efficiency in a combined cycle system including a gas turbine, a steam turbine, and a heat recovery steam generator, wherein gas turbine exhaust gas is used in the heat recovery steam generator for generating steam for the steam turbine, said gas turbine exhaust gas flowing from an entry end to an exit end of the heat recovery steam generator, the method comprising the steps of adding water to and heating fuel gas to produce heated, saturated fuel gas, the heat being derived from the heat recovery steam generator, feeding the saturated fuel gas to a fuel superheater; further heating the saturated fuel gas in the fuel superheater to superheat the fuel gas; and feeding the superheated, saturated fuel gas to the gas turbine. In a preferred implementation, the saturated fuel gas is also heated with heat derived from a heat source in the heat recovery steam generator.
The herein described modified bottoming cycle and method is applicable in particular to natural gas fire combined cycle applications.